1. Technical Field
The present invention relates in general to an improved fixture for restraining workpieces, and in particular to improving the flatness control of a workpiece during a lapping process. Still more particularly, the present invention relates to a compliant membrane for restraining a workpiece and applying uniform pressure thereto during a lapping process to improve flatness control thereof.
2. Description of the Prior Art
Data access and storage devices (DASDs) such as disk drives use magnetic recording heads to read data from or write data to the disks as they spin inside the drive. Each head has a polished air bearing surface (ABS) with flatness parameters, such as crown, camber, and twist. The ABS allows the head to xe2x80x9cflyxe2x80x9d above the surface of its respective spinning disk. In order to achieve the desired fly height, fly height variance, take-off speed, and other aerodynamic characteristics, the flatness parameters of the ABS need to be tightly controlled.
Although a number of processing steps are required to manufacture heads, the ABS flatness parameters are primarily determined during the final lapping process. The final lapping process may be performed on the heads after they have been separated or segmented into individual pieces, or on rows of heads prior to the segmentation step. This process requires the head or row to be restrained while an abrasive plate of specified curvature is rubbed against it. As the plate abrades the surface of the head, the abrasion process causes material removal on the head ABS and, in the optimum case, will cause the ABS to conform to the contour or curvature of the plate. The final lapping process also creates and defines the proper magnetic read sensor and write element material heights needed for magnetic recording.
There are a number of factors that affect the accuracy of ABS curvature during the final lapping process. These include diamond size/morphology, lubricant chemistry, lapping tangential surface velocity, plate material, lapping motion/path on the plate, and other lapping parameters. In addition to these parameters, three critical conditions must be satisfied. First, it is essential that the contour of the abrasive plate be tightly controlled since, in the best case, the ABS will conform to the curvature of the plate. In addition, all components of the process, including the head/row, must be restrained without distortion during lapping. Any variance in the restraining forces will cause the parts to distort and/or elastically deform upon removal of the forces. For example, if a head or row is lapped on an absolutely flat surface while it is clamped in a fixture, the part will elastically deform to a non-flat condition when it is released. The amount of deformation is proportional to the amount of elastic distortion created when the part was initially clamped.
A third condition affecting the accuracy of the ABS is the lapping force, which is the amount of force exerted by the abrasive plate on the part being lapped. Ideally, the lapping force is minimized to reduce distortion during the lapping process. The holding fixture exerts forces which are normal to the plate for pushing the part against the plate, and tangential to the plate for causing the part to slide over the plate for material removal. Unfortunately, this combination of forces elastically distorts the part (e.g., the head).
For example, to lap a flat surface on an initially curved ABS, the normal-directed force of the flat (and assumably non-deformable) plate against the curved ABS causes the ABS to temporarily flatten. The amount of deflection or flattening of the part will depend on the magnitude, direction, and distribution of the force on the part. Under sufficiently high normal-directed force, the entire surface area of the ABS is in contact with the plate. Introducing tangential movement of the part against an abrasive flat plate causes the entire surface area of the ABS to be abraded, not just the non-flat portions of the ABS. Upon removal of the normal-directed force, the ABS will elastically return to a non-flat condition. To minimize the amount of elastic return, it is desirable to provide a low but evenly distributed, normal-directed force on the part. The desired optimum low normal force will depend on a number of factors, such as diamond size/morphology, lubricant chemistry, lapping tangential velocity, and other lapping parameters. Thus, an improved apparatus and method for accurately defining the curvature of an ABS during the final lapping process is needed.
A lapping process fixture has a base with a mounting surface and a membrane on the mounting surface for supporting a workpiece. The membrane is bonded to the mounting surface and has adhesive on its outer surface such that the workpiece adheres to it. The membrane extends across and seals an opening to a cavity inside the base. The cavity is filled with fluid that may be sealed or externally pressurized through ports in the base. The fixture restrains the workpiece to minimize distortion of its surface during processing. The workpiece is restrained from normal-directed movement by fluidic pressure such that the normal force is uniformly distributed across the surface area of the workpiece. The external adhesive on the membrane restrains the workpiece from tangential movement.
Accordingly, it is an object of the present invention to provide an improved fixture for restraining workpieces.
It is an additional object of the present invention to improve the flatness control of a workpiece during a lapping process.
Still another object of the present invention is to provide a compliant membrane for restraining a workpiece and applying uniform pressure thereto during a lapping process to improve flatness control thereof.